Signal-injector

  
This unit is a single oscillator built around an LF351 JFET-input op amp. Resistors Rl and R2 bias the noninverting input while R3 biases the inverting input from the output. This layout provides lOO% negative feedback, but the decoupling caused by C2 gives reduced feedback and high-voltage gain when dealing with audio frequencies. The fundamental operatiog frequency is about 800 Hz. Potentiometer R4 is the output-level control. Th use it start at the speaker.
Signal-injector - schematic

Ifno tone is heard, move back to the amplifer input, and listen for the tone. Still if no tone is heard, continue backtracking from the output to the input, covering all stages in between. The stage where the signal is lost is the one that is not operating.




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